st: Interaction term for dummy variables

Hi All
I am running a negative binomial regression. Majority
of the independent variables are categorical for
example gender, ethnicity, occupation, backpain
(presence=1 and absence of back pain=0) etc. So I have
dummy coded these categorical variables. For example,
gender was dummy coded as Male. I also want to create
an interaction term between some of these dummy coded
variables. For example, gender and back pain.
Here's how I calculated the interaction term
Male*BackPain= Male x BackPain
ID Male BackPain Male*BackPain
1 1 0 0 (male without back pain)
2 1 0 0 (male without back pain)
3 0 1 0 (female with back pain)
4 0 0 0 female without backpain)
5 1 1 1 (male with back pain)
6 1 1 1 (male with back pain)
7 1 0 0 (male without back pain)
8 0 0 0 (female without backpain)
9 1 0 0 (male without back pain)
10 0 1 0 (female with back pain)
And I think the xi command i.e
xi i.gender*i.backpain
gives the same result.
However, I am concerned that if we calculate the
interaction terms by multiplying the two dummy coded
variables, in this case Male and BackPain, we are
combining some of the categories into the same group.
For example,in the above sample
ID=1 & Male*BackPain=0 represents male without back
pain, whereas
ID=3 & Male*BackPain=0 represents female with back
pain, and
ID=8 & Male*BackPain=0 represents female without back
pain.
Is that right or am I missing something trivial here?
I would really appreciate if someone can throw some
light on how to calculate the interaction terms
between dummy coded categorical variables.
Thank you for your time.
Regards
Anju
=====
____________________________________________________________
Anju Parthan,
Doctoral Candidate,
The University of Texas at Austin, PHAR-PHARMACY ADMIN
1 UNIVERSITY STATION A 1930, AUSTIN, TX-78712
Tel: (512) 459-4942; Fax: (512) 471-8762
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